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  <!--*********************************************************************-->
  <h1>"libc++" C++ Standard Library</h1>
  <!--*********************************************************************-->

  <p>libc++ is a new implementation of the C++ standard library, targeting
     C++11.</p>

  <p>All of the code in libc++ is <a
     href="http://llvm.org/docs/DeveloperPolicy.html#license">dual licensed</a>
     under the MIT license and the UIUC License (a BSD-like license).</p>

  <!--=====================================================================-->
  <h2 id="goals">Features and Goals</h2>
  <!--=====================================================================-->

    <ul>
        <li>Correctness as defined by the C++11 standard.</li>
        <li>Fast execution.</li>
        <li>Minimal memory use.</li>
        <li>Fast compile times.</li>
        <li>ABI compatibility with gcc's libstdc++ for some low-level features
            such as exception objects, rtti and memory allocation.</li>
        <li>Extensive unit tests.</li>
    </ul>

  <!--=====================================================================-->
  <h2 id="why">Why a new C++ Standard Library for C++11?</h2>
  <!--=====================================================================-->

  <p>After its initial introduction, many people have asked "why start a new
     library instead of contributing to an existing library?" (like Apache's
     libstdcxx, GNU's libstdc++, STLport, etc).  There are many contributing
     reasons, but some of the major ones are:</p>

  <ul>
  <li><p>From years of experience (including having implemented the standard
      library before), we've learned many things about implementing
      the standard containers which require ABI breakage and fundamental changes
      to how they are implemented.  For example, it is generally accepted that
      building std::string using the "short string optimization" instead of
      using Copy On Write (COW) is a superior approach for multicore
      machines (particularly in C++11, which has rvalue references).  Breaking
      ABI compatibility with old versions of the library was
      determined to be critical to achieving the performance goals of
      libc++.</p></li>

  <li><p>Mainline libstdc++ has switched to GPL3, a license which the developers
      of libc++ cannot use.  libstdc++ 4.2 (the last GPL2 version) could be
      independently extended to support C++11, but this would be a fork of the
      codebase (which is often seen as worse for a project than starting a new
      independent one).  Another problem with libstdc++ is that it is tightly
       integrated with G++ development, tending to be tied fairly closely to the
       matching version of G++.</p>
    </li>

  <li><p>STLport and the Apache libstdcxx library are two other popular
      candidates, but both lack C++11 support.  Our experience (and the
      experience of libstdc++ developers) is that adding support for C++11 (in
      particular rvalue references and move-only types) requires changes to
      almost every class and function, essentially amounting to a rewrite.
      Faced with a rewrite, we decided to start from scratch and evaluate every
      design decision from first principles based on experience.</p>

      <p>Further, both projects are apparently abandoned: STLport 5.2.1 was
      released in Oct'08, and STDCXX 4.2.1 in May'08.</p>

    </ul>

  <!--=====================================================================-->
  <h2 id="requirements">Platform Support</h2>
  <!--=====================================================================-->

   <p>libc++ is known to work on the following platforms, using g++-4.2 and
      clang (lack of C++11 language support disables some functionality).</p>

    <ul>
     <li>Mac OS X i386</li>
     <li>Mac OS X x86_64</li>
    </ul>

  <!--=====================================================================-->
  <h2 id="dir-structure">Current Status</h2>
  <!--=====================================================================-->

   <p>libc++ is a 100% complete C++11 implementation on Apple's OS X. </p>

   <p>
   Ports to other platforms are underway. Here are recent test
   results for <a href="results.Windows.html">Windows</a>
   and <a href="results.Linux.html">Linux</a>.
   </p>

  <!--=====================================================================-->
  <h2>Get it and get involved!</h2>
  <!--=====================================================================-->

  <p>First please review our
     <a href="http://llvm.org/docs/DeveloperPolicy.html">Developer's Policy</a>.

  <p>To check out the code, use:</p>

  <ul>
  <li><code>svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx</code></li>
  </ul>

  <p>
     On Mac OS 10.7 (Lion) and later, the easiest way to get this library is to install
     Xcode 4.2 or later.  However if you want to install tip-of-trunk from here
     (getting the bleeding edge), read on.  However, be warned that Mac OS
     10.7 will not boot without a valid copy of <code>libc++.1.dylib</code> in
     <code>/usr/lib</code>.
  </p>

  <p>
     Next:
  </p>
  
  <ul>
    <li><code>cd libcxx/lib</code></li>
    <li><code>export TRIPLE=-apple-</code></li>
    <li><code>./buildit</code></li>
    <li><code>ln -sf libc++.1.dylib libc++.dylib</code></li>
  </ul>
  
  <p>
     That should result in a libc++.1.dylib and libc++.dylib.  The safest thing
     to do is to use it from where your libcxx is installed instead of replacing
     these in your Mac OS.
  </p>

  <p>
  To use your system-installed libc++ with clang you can:
  </p>

  <ul>
    <li><code>clang++ -stdlib=libc++ test.cpp</code></li>
    <li><code>clang++ -std=c++11 -stdlib=libc++ test.cpp</code></li>
  </ul>

  <p>
  To use your tip-of-trunk libc++ on Mac OS with clang you can:
  </p>

  <ul>
    <li><code>export DYLD_LIBRARY_PATH=&lt;path-to-libcxx&gt;/lib</code>
    <li><code>clang++ -std=c++11 -stdlib=libc++ -nostdinc++
         -I&lt;path-to-libcxx&gt;/include -L&lt;path-to-libcxx&gt;/lib
         test.cpp</code></li>
  </ul>

  <p>To run the libc++ test suite (recommended):</p>

  <ul>
  <li><code>cd libcxx/test</code></li>
  <li><code>./testit</code></li>
     <ul>
       <li>You can alter the command line options <code>testit</code> uses
       with <code>export OPTIONS="whatever you need"</code></li>
     </ul>
  </ul>

  <!--=====================================================================-->
  <h3>Notes</h3>
  <!--=====================================================================-->

<p>
Building libc++ with <code>-fno-rtti</code> is not supported.  However linking
against it with <code>-fno-rtti</code> is supported.
</p>

  <p>Send discussions to the
  (<a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev">clang mailing list</a>).</p>

  <!--=====================================================================-->
  <h2>Design Documents</h2>
  <!--=====================================================================-->

<ul>
<li><a href="atomic_design.html"><tt>&lt;atomic&gt;</tt></a></li>
<li><a href="type_traits_design.html"><tt>&lt;type_traits&gt;</tt></a></li>
<li><a href="http://marshall.calepin.co/llvmclang-and-standard-libraries-on-mac-os-x.html">Excellent notes by Marshall Clow</a></li>
</ul>

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